The objective of this project is to further characterize the nature of host resistance to ectoparasite infestation, particularly with ticks. Studies examining the nature of resistance to louse infestation have centered on the use of immunosuppressants, athymic mice, and the genetics of the response. Attempts are made to adaptively transfer the resistance response. During the past year, evidence has been obtained to show that the alternate pathway of complement activation is important in the resistance response. Histamine levels have been shown to be elevated at the tick attachment on resistant animals; however, studies in which antihistamines with activity against H1 and H2 receptors were used indicate that histamine might not be important in the actual expression of resistance. Hosts resistant to tick infestation were protected against a challenge infestation with ticks infected with highly virulent Francisella tularensis. The role of the Langerhans cell in trapping tick antigens has also been examined. Studies on immunization against tick infestation are continuing with marked success. Studies on the immune response of cattle to tick infestation have been initiated.